Harvard University has selected insider Stephen Blyth, an investment manager with a focus on mathematical analysis, to lead the world’s largest endowment.

Blyth, who takes over the top job Jan. 1, holds a PhD in statistics from Harvard and recently wrote a book on quantitative finance. But he came up through the trading ranks on Wall Street and says he prefers to think of himself as “an investor first.’’

The board of the $36.4 billion endowment conducted a national search to find a replacement for chief executive Jane Mendillo, who is stepping down after six years. They picked one of Mendillo’s top lieutenants, who weathered the financial crisis and its aftermath with her.

Blyth joined the university’s investment arm, Harvard Management Co., in 2006 to build its bond team, which has performed well for the endowment. After Mendillo arrived in 2008, his portfolio grew and he was tasked with overseeing all the internal money managers at Harvard Management. In 2012, he was named managing director and head of public markets, overseeing managers inside Harvard as well as outside funds and firms hired by the endowment.

In a statement, Mendillo said she “can’t imagine a better choice to lead [the endowment] into the future. He has proven himself a talented and successful investor and a strong and effective leader.”

Blyth said he is on the same page as Mendillo on the way she has repositioned the portfolio since the financial crisis. “I feel we’re well-positioned now. I feel I’m inheriting a strong organization,’’ Blyth said.

But there is unspoken pressure to boost returns. Mendillo’s tenure was marked largely by unwinding risk and dumping bad investments, while moving into areas where she saw greater potential, such as direct real estate investments.

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Six years later, her numbers are in the middle of the pack — but not stellar, as the Harvard community and investment peers had come to expect.

On Tuesday, Harvard issued the final annual performance report of Mendillo’s tenure. The fund posted a gain of 15.4 percent during the fiscal year ended June 30, slightly below the 16.1 percent median for comparable endowments.

Blyth would not say whether he was under the gun to produce stronger results. Harvard depends on cash from the endowment to cover 36 percent of its annual budget.

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“I’m looking to build on that performance, and I’m looking to build on the portfolio that Jane has put in place,’’ he said.

Mendillo made a mark during her time at the helm by providing a steady hand during a difficult period and by forging closer relations with the university itself. It is a role that her predecessors did not always embrace, miles from the Cambridge campus in Harvard Management’s perch at the Federal Reserve tower in downtown Boston.

In navigating those relationships, Blyth can lean on his own academic experience. He has taught mathematics in the past at Imperial College London and in recent years has taught statistics at Harvard.

Blyth said his job won’t leave him time to teach this year. But “I’d love to get back in the classroom at some point,” he said.